1. Field of the Invention
The present invention relates generally to a piston assembly for an internal combustion engine, particularly for an automotive internal combustion engine. More specifically, the invention relates to a piston head block suitable for such an internal combustion engine which has an active lubrication system for injecting lubricant through an injection nozzle oriented in the engine cylinder in the vicinity of the bottom-dead-center (BDC) of the piston.
2. Description of the Background Art
Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 57-89809 discloses a of piston head block which is suitable for use in an automotive internal combustion engine with an active lubrication system. In the disclosed piston head block, a cut out is formed in the skirt section so as to receive therein a lubricant injecting nozzle in the vicinity of BDC of the piston stroke. Such an arrangement is effective for permitting the piston head to stroke beyond the position of the lubricant injection nozzle in the piston induction or combustion stroke. This allows the overall axial length of the engine cylinder to be made shorter for making the engine block height lower.
On the other hand, formation of a cut out in the skirt section of the piston head block forms a critical point on the piston for causing degradation of the fatigue resistance. Namely, though the prior proposed piston head block is formed with the cut-out with smoothly curved edge so as to minimize concentration of stress, it is still difficult to avoid concentration of stress during the piston stroke. Particularly, during piston stroke, transverse load perpendicular to the piston stroke axis is applied through the cone rod. This transverse force serves as a source of stress to cause cracking in the skirt at the portion where concentration of stress is caused.